The present invention is directed to methods and devices for avoiding unwanted longitudinal (common mode) currents in a subscriber loop of a telephone system, and more particularly to a method and device in which a longitudinal amplifier provides currents to current amplifiers that feed tip and ring lines of the subscriber loop to compensate for the longitudinal currents.
With reference to FIG. 1, a typical telephone system 10 includes a subscriber 12 that is a connected to a central station 14 through a subscriber line interface circuit 16 (SLIC) that performs various functions to make the connection. The subscriber 12 is connected to the SLIC 16 with a bidirectional balanced two wire connection, that is denoted as subscriber loop 18. The subscriber loop 18 is connected to the SLIC 16 at tip and ring terminals 20 that feed current amplifiers 22 that receive speech signals from a receiver 24 in the SLIC 16. The SLIC 16 connects the subscriber loop to the central station 14 through an unbalanced four wire connection 26.
Speech signals are transmitted through the telephone system as metallic (or normal mode) currents. However, unwanted interference, such as crosstalk from other telephone signals or noise related to power induction, may also be present in the subscriber loop. The present invention is directed to the removal of this interference that appears in the subscriber loop as a longitudinal (or common mode) current.
Various techniques for compensating for the longitudinal current have been attempted. For example, a common mode current feedback circuit may provide two compensating currents, one for each of the current amplifiers 22, to compensate for the longitudinal current. However, each of the compensating currents is provided to the current amplifiers through a single resistor. Since the difference between the input currents to the current amplifiers is not zero, a built in temperature sensitivity for the output currents results. Further, the noise introduced and power consumed by the known corrective techniques are higher than desired. See, U.S. Pat. No. 4,476,350 issued Oct. 9, 1984 to Aull, et at.
Accordingly, it is an object of the present invention to provide a novel device and method for compensating for longitudinal currents in the subscriber loop that obviates the problems of the prior art.
It is another object of the present invention to provide a novel device and method for compensating for longitudinal currents in the subscriber loop in which two pairs of matched currents are applied to two pairs of matched resistors at the current amplifiers to improve operating precision.
It is still another object of the present invention to provide a novel device and method for compensating for longitudinal currents in the subscriber loop in which the gain of the current amplifiers is reduced by one half, the gain of the longitudinal amplifier is increased from one to two, and the gain of the receiver is increased from one to two to reduce system noise in the SLIC.
It is a further object of the present invention to provide a novel device and method for compensating for longitudinal currents in the subscriber loop in which a differential current approach is used to increase power savings per chip area.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.